1. Field of the Invention
The invention relates generally to an electromagnetic wireless telemetry system used in a wellbore to communicate between equipment at the surface and a downhole tool positioned in the wellbore.
2. Background Art
A variety of techniques are currently used in the oilfield in order to communicate between the surface and downhole tools or sensors disposed in the wellbore. One such technique is wireless electromagnetic telemetry in which electromagnetic waves are transmitted through the earth and casing in the wellbore. Examples of electromagnetic telemetry systems are disclosed in U.S. Pat. Nos. 5,642,051 and 5,396,232, both of which are assigned to the assignee of the present invention.
FIG. 1 shows a schematic of an electromagnetic telemetry system. To communicate from the surface to a downhole tool 2 located in the wellbore 1 and connected to a tubing string 5, a modulated current is applied using a current source 7, which is electrically connected to a wellhead (not shown) and an electrically conductive ground member 13 staked into the ground 11 some distance away from the wellhead. A circuit is formed using the ground 11 and casing 3 disposed in the wellbore 1. To receive information from the surface, a processing unit 15 receives the signal in the form of a voltage difference between two points on the casing 3: an upper point 17 and a lower point 19. The measured voltage difference may be very small, for example, in the order of a microvolt. The processing unit 15 amplifies this signal, decodes it, and then communicates with the downhole tool 2, which takes action based on the received signal.
Sending information from the downhole tool 2 and the surface works in the reverse manner from that described above. The processing unit 15 may include a transmitter or transceiver to send signals to the surface. Current from the processing unit 15 is injected into the formation surrounding the casing 3 through two injection points on the casing 3, which can be the same points as the upper measuring point 17 and the lower measuring point 19. Specific coding of the current signal carries the information from downhole to the surface. At the surface, the current is measured as a voltage difference between the wellhead and a second point on the ground.
The two points on the casing must be separated by some amount of electrical resistance to provide a measurable voltage difference. One related art method is to provide an insulated gap in the tubing string between the two points, as shown in FIG. 2A. The insulated gap 21 is a section in the tubing where the upper part of the tubing is electrically insulated from the lower part of the tubing. It may be for example an insulated threaded connection in the tubing string 5. To receive a signal, the voltage difference is measured at an upper measuring point 23 above the insulated gap 21 and a lower measuring point 25 below the insulated gap 21. The tubing string 5 makes contact with the casing 3 above and below the insulated gap 21 at contact points 17 and 19, which are space apart. To transmit a signal, current can be injected at the same measuring points 23, 25. Electromagnetic telemetry systems using insulating gaps are disclosed in U.S. Pat. No. 7,080,699, which is assigned to the assignee of the present invention.
Another related art method is to provide two centralizers on the tubing string in order to provide two contact points with the casing some distance apart. Such a system is shown in FIG. 2B. An upper centralizer 27 and a lower centralizer 29 are disposed on the tubing string 5 and contact the casing 3 at points 17, 19. The upper measuring point 23 and the lower measuring point 25 are respectively located at the upper centralizer 27 and the lower centralizer 29 thus generating fixed measuring points. A cable (not shown) runs from the two measuring points 23, 25 to the processing unit used to communicate with the downhole tool. However, centralizers are expensive.